Underwater vehicles or devices using buoyancy control are currently used in oceans, lakes, and other bodies of water throughout the world to perform research, monitoring, and a variety of other tasks. Such vehicles generally cost significantly less to operate than large research ships for performing these tasks, while generally providing at least the same or better results. Buoyancy control systems can be used to guide these underwater vehicles to different depths and to maintain given depths within the respective ocean and/or lake. When using such systems, underwater vehicles must perform work (i.e., expend energy) in order to buoyantly ascend through water stratified in density as a result of temperature and/or salinity. For example, the range of seawater density variation arising from the natural oceanic range of temperature and salinity in the open ocean is less than 1%. A greater amount of energy must be expended to overcome water density differences induced by pressure when the underwater vehicle is less compressible (i.e., stiffer) than water. For example, the range of seawater density variation due to a pressure change from the sea surface to the sea floor in the open, deep ocean (e.g., 5-6 km depth) is approximately 2-3%.
Underwater vehicles or devices are generally fabricated from solid materials (e.g., metal, ceramic, or fiber/resin composites). Such vehicles are stiffer than and compress approximately half as much as seawater. Therefore, the energy required for underwater vehicles to ascend through the ocean can easily be dominated by the compressibility mismatch contribution to buoyancy. The same is true for shallow-diving vehicles in waters stratified by temperature and/or salinity. Compensation for a compressibility mismatch can be accomplished by incorporating a compliant part in a vehicle. For example, a pressure hull surrounding a spring-backed piston having a neutrally compressible float that tracks a parcel of seawater as it changes depth through ocean circulation can be used to closely match overall vehicle compressibility to the compressibility of seawater. Vehicles including spring-backed piston devices, however, are typically complex, expensive, and cumbersome.